PSI - Issue 35

A. Bovsunovsky et al. / Procedia Structural Integrity 35 (2022) 74–81 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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non-linear effects are highly sensitive to the closing crack. Their problem as applied to damage detection is the decrease of sensitivity with the increase of damping, Bovsunovsky and Surace (2001). A high level of damping can almost completely suppress the manifestation of non-linear effects. This phenomenon is demonstrated in Fig. 2 by the increase of logarithmic decrement of vibration in the range  =0.05-0.465.

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p/ 

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p/ 

Рис. 6.5. Амплитудно-частотные характеристики колебательной системы при вязком трении ( С о /C=0,723, m=1,  =0) : ( а) -   =5,0%; ( б) -   =15,1%; ( в) -   =46,5% The aim of the study was to demonstrate the effectiveness of higher harmonics method and damping characteristics for the detection of damage on the early stages in blades of aircraft gas turbine engines, to determine the conditions under which these methods are sensitive enough for the reliable flaw detection of blades, and to develop suitable experimental procedure of vibration-based damage detection of blades. 2. Research object and test equipment The object of the study was the working blades of a low-pressure compressor (titanium alloy VT-3-1) and a turbine (heat-resistant alloy steel EI-961) of a D-36 aircraft gas turbine engine. The appearance of blades is shown in Fig. 3. Mechanical properties of materials are given in Table 1 ( E is the elasticity modulus;  is the density;  u is the ultimate strength,  -1 is the fatigue limit). Fig. 2. Frequency dependences of the dynamic response factor of a structure with closing crack at different damping: ( а ) –  =0.05; ( b ) –  =0.15; ( c ) –  =0.465 ( p is the frequency of driving force;  is the natural frequency of structure). In addition, the problem of pseudo-superharmonic resonance caused by the non-linearity of driving force should be addressed, Bovsunovsky and Surace (2001). A crack significantly increases the dissipation of energy due to the appearance of plastic zone around its tip. In this case, according to Bovsunovsky (2004), the energy dissipated in a crack of mode I can be determined as a function of cyclic stress intensity factor range. Under certain conditions, the change of structural damping can be extremely sensitive to the presence of crack. To assess the effectiveness of damping for the reliable diagnostics of damage, it is necessary to consider several factors.

Fig. 3. Blades of D-36 aircraft gas turbine engine: (a) – alloy VT-3-1: (b) – alloy EI-961. (a) (b)